Variable Frequency Drives (VFD) have long been recognized as a viable way to reduce the energy consumed by 3 phase electric motors. Despite the proven history of this, many plant engineers and building operators have never taken the time to fully appreciate the tremendous cost savings that are possible.
First.. energy savings is not possible on all applications. Machines that require constant torque, such as conveyors and extruders, offer little in the way of energy savings by applying drives. Drives are routinely applied to these machines, but it’s for process improvement, not energy savings.
However, on centrifugal loads, such as fans and pumps, significant savings are possible because of the particular physics surrounding these loads. These are known as the affinity laws of pumps and fans, as illustrated by the figure on the right.
Note that power consumed by a motor driving a centrifugal load, will change in proportion to the cube of the change in speed. Simply put, this means that a small reduction in speed will produce an significant reduction in energy required by the motor. As an example, a 10% reduction in speed, will result in roughly a 27% reduction in power consumed.
Fans and pumps are usually sized for worst case scenarios, where the maximum flow is required. But in a normal operation, while that worst case scenario has to be considered in the design, it’s rarely needed, and running a motor at full speed when it isn’t needed is a huge waste of energy.
EMA has been involved in a number of drive installations, where the costs of the drive and installation were completely recovered in less than one year. We’ve been involved in extraordinary situations, where the payback was less than six months. Just recently, we were involved in a situation where the power savings on a large medium voltage motor was roughly $700,000 per year!
So the savings are real, the equipment is reliable, and it’s easy to install and maintain. No wonder that more and more VFD s are being applied to fan and pump loads.